Random Numbers part 2
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Random Numbers part 2
As for references on this subject, the one to turn to ﬁrst is Knuth [1]. Then try [2]. Only a few of the standard books on numerical methods [34] treat topics relating to random numbers.
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 7.1 Uniform Deviates 275 As for references on this subject, the one to turn to ﬁrst is Knuth [1]. Then try [2]. Only a few of the standard books on numerical methods [34] treat topics relating to random numbers. CITED REFERENCES AND FURTHER READING: Knuth, D.E. 1981, Seminumerical Algorithms, 2nd ed., vol. 2 of The Art of Computer Programming (Reading, MA: AddisonWesley), Chapter 3, especially §3.5. [1] visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) Bratley, P., Fox, B.L., and Schrage, E.L. 1983, A Guide to Simulation (New York: Springer Verlag). [2] Dahlquist, G., and Bjorck, A. 1974, Numerical Methods (Englewood Cliffs, NJ: PrenticeHall), Chapter 11. [3] Forsythe, G.E., Malcolm, M.A., and Moler, C.B. 1977, Computer Methods for Mathematical Computations (Englewood Cliffs, NJ: PrenticeHall), Chapter 10. [4] 7.1 Uniform Deviates Uniform deviates are just random numbers that lie within a speciﬁed range (typically 0 to 1), with any one number in the range just as likely as any other. They are, in other words, what you probably think “random numbers” are. However, we want to distinguish uniform deviates from other sorts of random numbers, for example numbers drawn from a normal (Gaussian) distribution of speciﬁed mean and standard deviation. These other sorts of deviates are almost always generated by performing appropriate operations on one or more uniform deviates, as we will see in subsequent sections. So, a reliable source of random uniform deviates, the subject of this section, is an essential building block for any sort of stochastic modeling or Monte Carlo computer work. SystemSupplied Random Number Generators Most C implementations have, lurking within, a pair of library routines for initializing, and then generating, “random numbers.” In ANSI C, the synopsis is: #include #define RAND_MAX ... void srand(unsigned seed); int rand(void); You initialize the random number generator by invoking srand(seed) with some arbitrary seed. Each initializing value will typically result in a different random sequence, or a least a different starting point in some one enormously long sequence. The same initializing value of seed will always return the same random sequence, however. You obtain successive random numbers in the sequence by successive calls to rand(). That function returns an integer that is typically in the range 0 to the largest representable positive value of type int (inclusive). Usually, as in ANSI C, this largest value is available as RAND_MAX, but sometimes you have to ﬁgure it out for yourself. If you want a random float value between 0.0 (inclusive) and 1.0 (exclusive), you get it by an expression like
 276 Chapter 7. Random Numbers x = rand()/(RAND_MAX+1.0); Now our ﬁrst, and perhaps most important, lesson in this chapter is: be very, very suspicious of a systemsupplied rand() that resembles the one just described. If all scientiﬁc papers whose results are in doubt because of bad rand()s were to disappear from library shelves, there would be a gap on each shelf about as visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) big as your ﬁst. Systemsupplied rand()s are almost always linear congruential generators, which generate a sequence of integers I1 , I2 , I3 , . . ., each between 0 and m − 1 (e.g., RAND_MAX) by the recurrence relation Ij+1 = aIj + c (mod m) (7.1.1) Here m is called the modulus, and a and c are positive integers called the multiplier and the increment respectively. The recurrence (7.1.1) will eventually repeat itself, with a period that is obviously no greater than m. If m, a, and c are properly chosen, then the period will be of maximal length, i.e., of length m. In that case, all possible integers between 0 and m − 1 occur at some point, so any initial “seed” choice of I0 is as good as any other: the sequence just takes off from that point. Although this general framework is powerful enough to provide quite decent random numbers, its implementation in many, if not most, ANSI C libraries is quite ﬂawed; quite a number of implementations are in the category “totally botched.” Blame should be apportioned about equally between the ANSI C committee and the implementors. The typical problems are these: First, since the ANSI standard speciﬁes that rand() return a value of type int — which is only a twobyte quantity on many machines — RAND_MAX is often not very large. The ANSI C standard requires only that it be at least 32767. This can be disastrous in many circumstances: for a Monte Carlo integration (§7.6 and §7.8), you might well want to evaluate 106 different points, but actually be evaluating the same 32767 points 30 times each, not at all the same thing! You should categorically reject any library random number routine with a twobyte returned value. Second, the ANSI committee’s published rationale includes the following mischievous passage: “The committee decided that an implementation should be allowed to provide a rand function which generates the best random sequence possible in that implementation, and therefore mandated no standard algorithm. It recognized the value, however, of being able to generate the same pseudorandom sequence in different implementations, and so it has published an example. . . . [emphasis added]” The “example” is unsigned long next=1; int rand(void) /* NOT RECOMMENDED (see text) */ { next = next*1103515245 + 12345; return (unsigned int)(next/65536) % 32768; } void srand(unsigned int seed) { next=seed; }
 7.1 Uniform Deviates 277 This corresponds to equation (7.1.1) with a = 1103515245, c = 12345, and m = 232 (since arithmetic done on unsigned long quantities is guaranteed to return the correct loworder bits). These are not particularly good choices for a and c, though they are not gross embarrassments by themselves. The real botches occur when implementors, taking the committee’s statement above as license, try to “improve” on the published example. For example, one popular 32bit PCcompatible compiler visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) provides a long generator that uses the above congruence, but swaps the highorder and loworder 16 bits of the returned value. Somebody probably thought that this extra ﬂourish added randomness; in fact it ruins the generator. While these kinds of blunders can, of course, be ﬁxed, there remains a fundamental ﬂaw in simple linear congruential generators, which we now discuss. The linear congruential method has the advantage of being very fast, requiring only a few operations per call, hence its almost universal use. It has the disadvantage that it is not free of sequential correlation on successive calls. If k random numbers at a time are used to plot points in k dimensional space (with each coordinate between 0 and 1), then the points will not tend to “ﬁll up” the kdimensional space, but rather will lie on (k − 1)dimensional “planes.” There will be at most about m1/k such planes. If the constants m, a, and c are not very carefully chosen, there will be many fewer than that. If m is as bad as 32768, then the number of planes on which triples of points lie in threedimensional space will be no greater than about the cube root of 32768, or 32. Even if m is close to the machine’s largest representable integer, e.g., ∼ 232 , the number of planes on which triples of points lie in threedimensional space is usually no greater than about the cube root of 232, about 1600. You might well be focusing attention on a physical process that occurs in a small fraction of the total volume, so that the discreteness of the planes can be very pronounced. Even worse, you might be using a generator whose choices of m, a, and c have been botched. One infamous such routine, RANDU, with a = 65539 and m = 231 , was widespread on IBM mainframe computers for many years, and widely copied onto other systems [1]. One of us recalls producing a “random” plot with only 11 planes, and being told by his computer center’s programming consultant that he had misused the random number generator: “We guarantee that each number is random individually, but we don’t guarantee that more than one of them is random.” Figure that out. Correlation in kspace is not the only weakness of linear congruential generators. Such generators often have their loworder (least signiﬁcant) bits much less random than their highorder bits. If you want to generate a random integer between 1 and 10, you should always do it using highorder bits, as in j=1+(int) (10.0*rand()/(RAND_MAX+1.0)); and never by anything resembling j=1+(rand() % 10); (which uses lowerorder bits). Similarly you should never try to take apart a “rand()” number into several supposedly random pieces. Instead use separate calls for every piece.
 278 Chapter 7. Random Numbers Portable Random Number Generators Park and Miller [1] have surveyed a large number of random number generators that have been used over the last 30 years or more. Along with a good theoretical review, they present an anecdotal sampling of a number of inadequate generators that have come into widespread use. The historical record is nothing if not appalling. visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) There is good evidence, both theoretical and empirical, that the simple mul tiplicative congruential algorithm Ij+1 = aIj (mod m) (7.1.2) can be as good as any of the more general linear congruential generators that have c = 0 (equation 7.1.1) — if the multiplier a and modulus m are chosen exquisitely carefully. Park and Miller propose a “Minimal Standard” generator based on the choices a = 75 = 16807 m = 231 − 1 = 2147483647 (7.1.3) First proposed by Lewis, Goodman, and Miller in 1969, this generator has in subsequent years passed all new theoretical tests, and (perhaps more importantly) has accumulated a large amount of successful use. Park and Miller do not claim that the generator is “perfect” (we will see below that it is not), but only that it is a good minimal standard against which other generators should be judged. It is not possible to implement equations (7.1.2) and (7.1.3) directly in a highlevel language, since the product of a and m − 1 exceeds the maximum value for a 32bit integer. Assembly language implementation using a 64bit product register is straightforward, but not portable from machine to machine. A trick due to Schrage [2,3] for multiplying two 32bit integers modulo a 32bit constant, without using any intermediates larger than 32 bits (including a sign bit) is therefore extremely interesting: It allows the Minimal Standard generator to be implemented in essentially any programming language on essentially any machine. Schrage’s algorithm is based on an approximate factorization of m, m = aq + r, i.e., q = [m/a], r = m mod a (7.1.4) with square brackets denoting integer part. If r is small, speciﬁcally r < q, and 0 < z < m − 1, it can be shown that both a(z mod q) and r[z/q] lie in the range 0, . . . , m − 1, and that a(z mod q) − r[z/q] if it is ≥ 0, az mod m = (7.1.5) a(z mod q) − r[z/q] + m otherwise The application of Schrage’s algorithm to the constants (7.1.3) uses the values q = 127773 and r = 2836. Here is an implementation of the Minimal Standard generator:
 7.1 Uniform Deviates 279 #define IA 16807 #define IM 2147483647 #define AM (1.0/IM) #define IQ 127773 #define IR 2836 #define MASK 123459876 float ran0(long *idum) visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) “Minimal” random number generator of Park and Miller. Returns a uniform random deviate between 0.0 and 1.0. Set or reset idum to any integer value (except the unlikely value MASK) to initialize the sequence; idum must not be altered between calls for successive deviates in a sequence. { long k; float ans; *idum ^= MASK; XORing with MASK allows use of zero and other k=(*idum)/IQ; simple bit patterns for idum. *idum=IA*(*idumk*IQ)IR*k; Compute idum=(IA*idum) % IM without over if (*idum < 0) *idum += IM; ﬂows by Schrage’s method. ans=AM*(*idum); Convert idum to a ﬂoating result. *idum ^= MASK; Unmask before return. return ans; } The period of ran0 is 231 − 2 ≈ 2.1 × 109 . A peculiarity of generators of the form (7.1.2) is that the value 0 must never be allowed as the initial seed — it perpetuates itself — and it never occurs for any nonzero initial seed. Experience has shown that users always manage to call random number generators with the seed idum=0. That is why ran0 performs its exclusiveor with an arbitrary constant both on entry and exit. If you are the ﬁrst user in history to be proof against human error, you can remove the two lines with the ∧ operation. Park and Miller discuss two other multipliers a that can be used with the same m = 231 − 1. These are a = 48271 (with q = 44488 and r = 3399) and a = 69621 (with q = 30845 and r = 23902). These can be substituted in the routine ran0 if desired; they may be slightly superior to Lewis et al.’s longertested values. No values other than these should be used. The routine ran0 is a Minimal Standard, satisfactory for the majority of applications, but we do not recommend it as the ﬁnal word on random number generators. Our reason is precisely the simplicity of the Minimal Standard. It is not hard to think of situations where successive random numbers might be used in a way that accidentally conﬂicts with the generation algorithm. For example, since successive numbers differ by a multiple of only 1.6 × 104 out of a modulus of more than 2 × 109 , very small random numbers will tend to be followed by smaller than average values. One time in 106 , for example, there will be a value < 10−6 returned (as there should be), but this will always be followed by a value less than about 0.0168. One can easily think of applications involving rare events where this property would lead to wrong results. There are other, more subtle, serial correlations present in ran0. For example, if successive points (Ii , Ii+1 ) are binned into a twodimensional plane for i = 1, 2, . . . , N , then the resulting distribution fails the χ2 test when N is greater than a few ×107 , much less than the period m − 2. Since loworder serial correlations have historically been such a bugaboo, and since there is a very simple way to remove
 280 Chapter 7. Random Numbers them, we think that it is prudent to do so. The following routine, ran1, uses the Minimal Standard for its random value, but it shufﬂes the output to remove loworder serial correlations. A random deviate derived from the jth value in the sequence, Ij , is output not on the jth call, but rather on a randomized later call, j + 32 on average. The shufﬂing algorithm is due to Bays and Durham as described in Knuth [4], and is illustrated in Figure 7.1.1. visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) #define IA 16807 #define IM 2147483647 #define AM (1.0/IM) #define IQ 127773 #define IR 2836 #define NTAB 32 #define NDIV (1+(IM1)/NTAB) #define EPS 1.2e7 #define RNMX (1.0EPS) float ran1(long *idum) “Minimal” random number generator of Park and Miller with BaysDurham shuﬄe and added safeguards. Returns a uniform random deviate between 0.0 and 1.0 (exclusive of the endpoint values). Call with idum a negative integer to initialize; thereafter, do not alter idum between successive deviates in a sequence. RNMX should approximate the largest ﬂoating value that is less than 1. { int j; long k; static long iy=0; static long iv[NTAB]; float temp; if (*idum =0;j) { Load the shuﬄe table (after 8 warmups). k=(*idum)/IQ; *idum=IA*(*idumk*IQ)IR*k; if (*idum < 0) *idum += IM; if (j < NTAB) iv[j] = *idum; } iy=iv[0]; } k=(*idum)/IQ; Start here when not initializing. *idum=IA*(*idumk*IQ)IR*k; Compute idum=(IA*idum) % IM without over if (*idum < 0) *idum += IM; ﬂows by Schrage’s method. j=iy/NDIV; Will be in the range 0..NTAB1. iy=iv[j]; Output previously stored value and reﬁll the iv[j] = *idum; shuﬄe table. if ((temp=AM*iy) > RNMX) return RNMX; Because users don’t expect endpoint values. else return temp; } The routine ran1 passes those statistical tests that ran0 is known to fail. In fact, we do not know of any statistical test that ran1 fails to pass, except when the number of calls starts to become on the order of the period m, say > 108 ≈ m/20. For situations when even longer random sequences are needed, L’Ecuyer [6] has given a good way of combining two different sequences with different periods so as to obtain a new sequence whose period is the least common multiple of the two periods. The basic idea is simply to add the two sequences, modulo the modulus of
 7.1 Uniform Deviates 281 iy 1 visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) iv0 OUTPUT RAN 3 2 iv31 Figure 7.1.1. Shufﬂing procedure used in ran1 to break up sequential correlations in the Minimal Standard generator. Circled numbers indicate the sequence of events: On each call, the random number in iy is used to choose a random element in the array iv. That element becomes the output random number, and also is the next iy. Its spot in iv is reﬁlled from the Minimal Standard routine. either of them (call it m). A trick to avoid an intermediate value that overﬂows the integer wordsize is to subtract rather than add, and then add back the constant m − 1 if the result is ≤ 0, so as to wrap around into the desired interval 0, . . . , m − 1. Notice that it is not necessary that this wrapped subtraction be able to reach all values 0, . . . , m − 1 from every value of the ﬁrst sequence. Consider the absurd extreme case where the value subtracted was only between 1 and 10: The resulting sequence would still be no less random than the ﬁrst sequence by itself. As a practical matter it is only necessary that the second sequence have a range covering substantially all of the range of the ﬁrst. L’Ecuyer recommends the use of the two generators m1 = 2147483563 (with a1 = 40014, q1 = 53668, r1 = 12211) and m2 = 2147483399 (with a2 = 40692, q2 = 52774, r2 = 3791). Both moduli are slightly less than 231 . The periods m1 − 1 = 2 × 3 × 7 × 631 × 81031 and m2 − 1 = 2 × 19 × 31 × 1019 × 1789 share only the factor 2, so the period of the combined generator is ≈ 2.3 × 1018 . For present computers, period exhaustion is a practical impossibility. Combining the two generators breaks up serial correlations to a considerable extent. We nevertheless recommend the additional shufﬂe that is implemented in the following routine, ran2. We think that, within the limits of its ﬂoatingpoint precision, ran2 provides perfect random numbers; a practical deﬁnition of “perfect” is that we will pay $1000 to the ﬁrst reader who convinces us otherwise (by ﬁnding a statistical test that ran2 fails in a nontrivial way, excluding the ordinary limitations of a machine’s ﬂoatingpoint representation).
 282 Chapter 7. Random Numbers #define IM1 2147483563 #define IM2 2147483399 #define AM (1.0/IM1) #define IMM1 (IM11) #define IA1 40014 #define IA2 40692 #define IQ1 53668 #define IQ2 52774 visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) #define IR1 12211 #define IR2 3791 #define NTAB 32 #define NDIV (1+IMM1/NTAB) #define EPS 1.2e7 #define RNMX (1.0EPS) float ran2(long *idum) Long period (> 2 × 1018) random number generator of L’Ecuyer with BaysDurham shuﬄe and added safeguards. Returns a uniform random deviate between 0.0 and 1.0 (exclusive of the endpoint values). Call with idum a negative integer to initialize; thereafter, do not alter idum between successive deviates in a sequence. RNMX should approximate the largest ﬂoating value that is less than 1. { int j; long k; static long idum2=123456789; static long iy=0; static long iv[NTAB]; float temp; if (*idum =0;j) { Load the shuﬄe table (after 8 warmups). k=(*idum)/IQ1; *idum=IA1*(*idumk*IQ1)k*IR1; if (*idum < 0) *idum += IM1; if (j < NTAB) iv[j] = *idum; } iy=iv[0]; } k=(*idum)/IQ1; Start here when not initializing. *idum=IA1*(*idumk*IQ1)k*IR1; Compute idum=(IA1*idum) % IM1 without if (*idum < 0) *idum += IM1; overﬂows by Schrage’s method. k=idum2/IQ2; idum2=IA2*(idum2k*IQ2)k*IR2; Compute idum2=(IA2*idum) % IM2 likewise. if (idum2 < 0) idum2 += IM2; j=iy/NDIV; Will be in the range 0..NTAB1. iy=iv[j]idum2; Here idum is shuﬄed, idum and idum2 are iv[j] = *idum; combined to generate output. if (iy < 1) iy += IMM1; if ((temp=AM*iy) > RNMX) return RNMX; Because users don’t expect endpoint values. else return temp; } L’Ecuyer [6] lists additional short generators that can be combined into longer ones, including generators that can be implemented in 16bit integer arithmetic. Finally, we give you Knuth’s suggestion [4] for a portable routine, which we have translated to the present conventions as ran3. This is not based on the linear congruential method at all, but rather on a subtractive method (see also [5]). One might hope that its weaknesses, if any, are therefore of a highly different character
 7.1 Uniform Deviates 283 from the weaknesses, if any, of ran1 above. If you ever suspect trouble with one routine, it is a good idea to try the other in the same application. ran3 has one nice feature: if your machine is poor on integer arithmetic (i.e., is limited to 16bit integers), you can declare mj, mk, and ma[] as float, deﬁne mbig and mseed as 4000000 and 1618033, respectively, and the routine will be rendered entirely ﬂoatingpoint. visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) #include Change to math.h in K&R C. #define MBIG 1000000000 #define MSEED 161803398 #define MZ 0 #define FAC (1.0/MBIG) According to Knuth, any large MBIG, and any smaller (but still large) MSEED can be substituted for the above values. float ran3(long *idum) Returns a uniform random deviate between 0.0 and 1.0. Set idum to any negative value to initialize or reinitialize the sequence. { static int inext,inextp; static long ma[56]; The value 56 (range ma[1..55]) is special and static int iff=0; should not be modiﬁed; see Knuth. long mj,mk; int i,ii,k; if (*idum < 0  iff == 0) { Initialization. iff=1; mj=labs(MSEEDlabs(*idum)); Initialize ma[55] using the seed idum and the mj %= MBIG; large number MSEED. ma[55]=mj; mk=1; for (i=1;i
 284 Chapter 7. Random Numbers process data from an experiment not always in exactly the same order, for example, so that the ﬁrst output is more “typical” than might otherwise be the case. For this kind of application, all we really need is a list of “good” choices for m, a, and c in equation (7.1.1). If we don’t need a period longer than 104 to 106 , say, we can keep the value of (m − 1)a + c small enough to avoid overﬂows that would otherwise mandate the extra complexity of Schrage’s method (above). We can thus easily embed in our programs visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) unsigned long jran,ia,ic,im; float ran; ... jran=(jran*ia+ic) % im; ran=(float) jran / (float) im; whenever we want a quick and dirty uniform deviate, or jran=(jran*ia+ic) % im; j=jlo+((jhijlo+1)*jran)/im; whenever we want an integer between jlo and jhi, inclusive. (In both cases jran was once initialized to any seed value between 0 and im1.) Be sure to remember, however, that when im is small, the kth root of it, which is the number of planes in kspace, is even smaller! So a quick and dirty generator should never be used to select points in kspace with k > 1. With these caveats, some “good” choices for the constants are given in the accompanying table. These constants (i) give a period of maximal length im, and, more important, (ii) pass Knuth’s “spectral√ test” for dimensions 2, 3, 4, 5, and 6. The increment ic is a prime, close to the value ( 1 − 1 3)im; actually almost any value of ic that is relatively prime to im will do 2 6 just as well, but there is some “lore” favoring this choice (see [4], p. 84). An Even Quicker Generator In C, if you multiply two unsigned long int integers on a machine with a 32bit long integer representation, the value returned is the loworder 32 bits of the true 64bit product. If we now choose m = 232 , the “mod” in equation (7.1.1) is free, and we have simply Ij+1 = aIj + c (7.1.6) Knuth suggests a = 1664525 as a suitable multiplier for this value of m. H.W. Lewis conducted extensive tests of this value of a with c = 1013904223, which is a prime close has √ to ( 5 − 2)m. The resulting inline generator (we will call it ranqd1) is simply unsigned long idum; ... idum = 1664525L*idum + 1013904223L; This is about as good as any 32bit linear congruential generator, entirely adequate for many uses. And, with only a single multiply and add, it is very fast. To check whether your machine has the desired integer properties, see if you can generate the following sequence of 32bit values (given here in hex): 00000000, 3C6EF35F, 47502932, D1CCF6E9, AAF95334, 6252E503, 9F2EC686, 57FE6C2D, A3D95FA8, 81FD BEE7, 94F0AF1A, CBF633B1. If you need ﬂoatingpoint values instead of 32bit integers, and want to avoid a divide by ﬂoatingpoint 232 , a dirty trick is to mask in an exponent that makes the value lie between 1 and 2, then subtract 1.0. The resulting inline generator (call it ranqd2) will look something like
 7.1 Uniform Deviates 285 Constants for Quick and Dirty Random Number Generators overﬂow at im ia ic overﬂow at im ia ic 6075 106 1283 86436 1093 18257 220 121500 1021 25673 7875 211 1663 259200 421 54773 visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) 221 227 7875 421 1663 117128 1277 24749 222 121500 2041 25673 6075 1366 1283 312500 741 66037 6655 936 1399 228 11979 430 2531 145800 3661 30809 223 175000 2661 36979 14406 967 3041 233280 1861 49297 29282 419 6173 244944 1597 51749 53125 171 11213 229 24 2 139968 3877 29573 12960 1741 2731 214326 3613 45289 14000 1541 2957 714025 1366 150889 21870 1291 4621 230 31104 625 6571 134456 8121 28411 139968 205 29573 259200 7141 54773 225 231 29282 1255 6173 233280 9301 49297 81000 421 17117 714025 4096 150889 134456 281 28411 232 226 unsigned long idum,itemp; float rand; #ifdef vax static unsigned long jflone = 0x00004080; static unsigned long jflmsk = 0xffff007f; #else static unsigned long jflone = 0x3f800000; static unsigned long jflmsk = 0x007fffff; #endif ... idum = 1664525L*idum + 1013904223L; itemp = jflone  (jflmsk & idum); rand = (*(float *)&itemp)1.0; The hex constants 3F800000 and 007FFFFF are the appropriate ones for computers using the IEEE representation for 32bit ﬂoatingpoint numbers (e.g., IBM PCs and most UNIX workstations). For DEC VAXes, the correct hex constants are, respectively, 00004080 and FFFF007F. Notice that the IEEE mask results in the ﬂoatingpoint number being constructed out of the 23 loworder bits of the integer, which is not ideal. (Your authors have tried very hard to make almost all of the material in this book machine and compiler independent — indeed, even programming language independent. This subsection is a rare aberration. Forgive us. Once in a great while the temptation to be really dirty is just irresistible.) Relative Timings and Recommendations Timings are inevitably machine dependent. Nevertheless the following table is indicative of the relative timings, for typical machines, of the various uniform
 286 Chapter 7. Random Numbers generators discussed in this section, plus ran4 from §7.5. Smaller values in the table indicate faster generators. The generators ranqd1 and ranqd2 refer to the “quick and dirty” generators immediately above. Generator Relative Execution Time visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) ran0 ≡ 1.0 ran1 ≈ 1.3 ran2 ≈ 2.0 ran3 ≈ 0.6 ranqd1 ≈ 0.10 ranqd2 ≈ 0.25 ran4 ≈ 4.0 On balance, we recommend ran1 for general use. It is portable, based on Park and Miller’s Minimal Standard generator with an additional shufﬂe, and has no known (to us) ﬂaws other than period exhaustion. If you are generating more than 100,000,000 random numbers in a single calculation (that is, more than about 5% of ran1’s period), we recommend the use of ran2, with its much longer period. Knuth’s subtractive routine ran3 seems to be the timing winner among portable routines. Unfortunately the subtractive method is not so well studied, and not a standard. We like to keep ran3 in reserve for a “second opinion,” substituting it when we suspect another generator of introducing unwanted correlations into a calculation. The routine ran4 generates extremely good random deviates, and has some other nice properties, but it is slow. See §7.5 for discussion. Finally, the quick and dirty inline generators ranqd1 and ranqd2 are very fast, but they are somewhat machine dependent, and at best only as good as a 32bit linear congruential generator ever is — in our view not good enough in many situations. We would use these only in very special cases, where speed is critical. CITED REFERENCES AND FURTHER READING: Park, S.K., and Miller, K.W. 1988, Communications of the ACM, vol. 31, pp. 1192–1201. [1] Schrage, L. 1979, ACM Transactions on Mathematical Software, vol. 5, pp. 132–138. [2] Bratley, P., Fox, B.L., and Schrage, E.L. 1983, A Guide to Simulation (New York: Springer Verlag). [3] Knuth, D.E. 1981, Seminumerical Algorithms, 2nd ed., vol. 2 of The Art of Computer Programming (Reading, MA: AddisonWesley), §§3.2–3.3. [4] Kahaner, D., Moler, C., and Nash, S. 1989, Numerical Methods and Software (Englewood Cliffs, NJ: Prentice Hall), Chapter 10. [5] L’Ecuyer, P. 1988, Communications of the ACM, vol. 31, pp. 742–774. [6] Forsythe, G.E., Malcolm, M.A., and Moler, C.B. 1977, Computer Methods for Mathematical Computations (Englewood Cliffs, NJ: PrenticeHall), Chapter 10.
 7.2 Transformation Method: Exponential and Normal Deviates 287 7.2 Transformation Method: Exponential and Normal Deviates In the previous section, we learned how to generate random deviates with a uniform probability distribution, so that the probability of generating a number visit website http://www.nr.com or call 18008727423 (North America only),or send email to trade@cup.cam.ac.uk (outside North America). readable files (including this one) to any servercomputer, is strictly prohibited. To order Numerical Recipes books,diskettes, or CDROMs Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copying of machine Copyright (C) 19881992 by Cambridge University Press.Programs Copyright (C) 19881992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0521431085) between x and x + dx, denoted p(x)dx, is given by dx 0 < x < 1 p(x)dx = (7.2.1) 0 otherwise The probability distribution p(x) is of course normalized, so that ∞ p(x)dx = 1 (7.2.2) −∞ Now suppose that we generate a uniform deviate x and then take some prescribed function of it, y(x). The probability distribution of y, denoted p(y)dy, is determined by the fundamental transformation law of probabilities, which is simply p(y)dy = p(x)dx (7.2.3) or dx p(y) = p(x) (7.2.4) dy Exponential Deviates As an example, suppose that y(x) ≡ − ln(x), and that p(x) is as given by equation (7.2.1) for a uniform deviate. Then dx p(y)dy = dy = e−y dy (7.2.5) dy which is distributed exponentially. This exponential distribution occurs frequently in real problems, usually as the distribution of waiting times between independent Poissonrandom events, for example the radioactive decay of nuclei. You can also easily see (from 7.2.4) that the quantity y/λ has the probability distribution λe−λy . So we have #include float expdev(long *idum) Returns an exponentially distributed, positive, random deviate of unit mean, using ran1(idum) as the source of uniform deviates. { float ran1(long *idum); float dum; do dum=ran1(idum); while (dum == 0.0); return log(dum); }
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